Nitride semiconductor light-emitting element and method for producing same

ABSTRACT

Disclosed is a nitride semiconductor light-emitting element comprising a p-type nitride semiconductor layer 1, a p-type nitride semiconductor layer 2, and a p-type nitride semiconductor layer 3 placed in order above a nitride semiconductor active layer, wherein the p-type nitride semiconductor layer 1 and p-type nitride semiconductor layer 2 each contain Al, the average Al composition of the p-type nitride semiconductor layer 1 is equivalent to the average Al composition of the p-type nitride semiconductor layer 2, the p-type nitride semiconductor layer 3 has a smaller band gap than the p-type nitride semiconductor layer 2, the p-type impurity concentration of the p-type nitride semiconductor layer 2 and the p-type impurity concentration of the p-type nitride semiconductor layer 3 are both lower than the p-type impurity concentration of the p-type nitride semiconductor layer 1, and a method for producing same.

REFERENCE TO RELATED APPLICATIONS

This application is a national stage application under 35 USC 371 ofInternational Application No. PCT/JP2011/053358, filed Feb. 17, 2011,which claims the priority of Japanese Patent Application No.2010-034919, filed Feb. 19, 2010, the entire contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a nitride semiconductor light-emittingelement and a method for producing a nitride semiconductorlight-emitting element.

BACKGROUND OF THE INVENTION

A nitride semiconductor light-emitting element prepared by employing anAlGaInN-based nitride semiconductor is capable of emittingshort-wavelength light such as blue light in high efficiency, and hencea light-emitting device emitting white light can be obtained bycombining the same with a fluorescent material. A light-emitting deviceoverstriding luminous efficiency a fluorescent lamp is increasinglyobtained as a light-emitting device emitting white light, and hence sucha light-emitting device is considered as playing the leading part infuture illumination.

As to a light-emitting device emitting white light with such a nitridesemiconductor light-emitting element, on the other hand, furtherimprovement of luminous efficiency and resulting development of energysaving are expected.

The principle of light emission in the nitride semiconductorlight-emitting element resides in recombination of holes and electrons,and hence it is important to properly prepare a p-type nitridesemiconductor layer and an n-type nitride semiconductor layer. In aproduction process for the nitride semiconductor light-emitting element,however, there are various heat treatment steps disturbing distributionof a p-type impurity such as Mg doped into the p-type nitridesemiconductor layer.

First, there is an epitaxial growth step of forming the p-type nitridesemiconductor layer itself, which is generally carried out at a hightemperature exceeding 1000° C. Second, there is an annealing step forprompting control of the p-type nitride semiconductor layer to thep-type. Third, there is a heat treatment step for improving contactnessbetween an electrode and a nitride semiconductor and the characteristicsof the electrode itself after formation of the electrode.

As a result of the heat history in these heat treatment steps,difference arises between the doping quantity of the p-type impurityduring epitaxial growth of the p-type nitride semiconductor layer andthe doping profile of the p-type nitride semiconductor layer obtained bythe doping of the p-type impurity.

Various technical developments for improving the characteristics of anitride semiconductor light-emitting element by solving such a problemare conducted.

In PTL 1 (Japanese Patent Laying-Open No. 2009-130097), for example,there is disclosed a technique of forming a multilayer structure of anMg-doped p-type Al_(0.15)Ga_(0.85)N blocking layer and an Mg-dopedp-type GaN layer on an active region.

In PTL 2 (Japanese Patent Laying-Open No. 2000-164922), for example,there is disclosed a technique of improving contactness with a positiveelectrode by increasing an Mg impurity concentration in an outermostportion of a p-type contact layer in contact with the positiveelectrode.

In PTL 3 (Japanese Patent Laying-Open No. 2001-148507), further, thereis disclosed a technique of bringing a p-type nitride semiconductorlayer on an active layer into a structure obtained by stacking threelayers including a p-type cladding layer doped with a p-type impurity ina middle concentration, a p-type low-concentration doping layer dopedwith the p-type impurity in a low concentration and a p-type contactlayer doped with the p-type impurity in a high concentration in thisorder.

In Example 7 of PTL 3, there is disclosed a technique of forming ap-type cladding layer made of p-type Al_(0.16)Ga_(0.85)N doped with Mgby 5×10¹⁹ atoms/cm³ as a p-type layer doped with Mg in a middleconcentration, forming a low-concentration doping layer made of undopedGaN as a p-type layer doped with Mg in a low concentration and forming ahigh-concentration doping layer doped with Mg by 1×10²⁰ atoms/cm³ as ap-type layer doped with Mg in a high concentration.

-   PTL 1: Japanese Patent Laying-Open No. 2009-130097-   PTL 2: Japanese Patent Laying-Open No. 2000-164922-   PTL 3: Japanese Patent Laying-Open No. 2001-148507

SUMMARY OF THE INVENTION

However, further improvement of the characteristics of a nitridesemiconductor light-emitting element is required, due to the recentraising of consciousness about environmental problems.

In consideration of the aforementioned circumstances, an object of thepresent invention is to provide a nitride semiconductor light-emittingelement excellent in characteristics and a method for producing anitride semiconductor light-emitting element.

The present invention provides a nitride semiconductor light-emittingelement including an n-type nitride semiconductor layer, a nitridesemiconductor active layer provided on the n-type nitride semiconductorlayer and a p-type nitride semiconductor layer provided on the nitridesemiconductor active layer, in which the p-type nitride semiconductorlayer includes a first p-type nitride semiconductor layer, a secondp-type nitride semiconductor layer and a third p-type nitridesemiconductor layer in this order from the side of the nitridesemiconductor active layer, the first p-type nitride semiconductor layerand the second p-type nitride semiconductor layer contain Al (aluminum)respectively, an average Al composition in the first p-type nitridesemiconductor layer and an average Al composition in the second p-typenitride semiconductor layer are equivalent to each other, the thirdp-type nitride semiconductor layer has a smaller band gap than thesecond p-type nitride semiconductor layer, and the p-type impurityconcentration in the second p-type nitride semiconductor layer and thep-type impurity concentration in the third p-type nitride semiconductorlayer are lower than the p-type impurity concentration in the firstp-type nitride semiconductor layer respectively.

Preferably in the nitride semiconductor light-emitting element accordingto the present invention, the p-type nitride semiconductor layer furtherincludes a fourth p-type nitride semiconductor layer on a side of thethird p-type nitride semiconductor layer opposite to the side where thenitride semiconductor active layer is set, the fourth p-type nitridesemiconductor layer has a smaller band gap than the second p-typenitride semiconductor layer, and the p-type impurity concentration inthe fourth p-type nitride semiconductor layer is higher than the p-typeimpurity concentration in the third p-type nitride semiconductor layer.

Preferably in the nitride semiconductor light-emitting element accordingto the present invention, the nitride semiconductor active layer has amultiple quantum well structure including a plurality of nitridesemiconductor quantum well layers and a plurality of nitridesemiconductor barrier layers, and a nitride semiconductor barrier layer,included in the plurality of nitride semiconductor barrier layers, otherthan a nitride semiconductor barrier layer in contact with the p-typenitride semiconductor layer contains an n-type impurity.

Preferably in the nitride semiconductor light-emitting element accordingto the present invention, the n-type nitride semiconductor layerincludes an n-type nitride semiconductor contact layer and an n-typenitride semiconductor superlattice layer, the n-type nitridesemiconductor superlattice layer is positioned between the n-typenitride semiconductor contact layer and the nitride semiconductor activelayer, and an average n-type impurity concentration in the n-typenitride semiconductor superlattice layer is at least 1×10¹⁸ atoms/cm³.

The present invention further provides a method for producing a nitridesemiconductor light-emitting element, including the steps ofvapor-phase-growing a nitride semiconductor active layer on an n-typenitride semiconductor layer, vapor-phase-growing a first p-type nitridesemiconductor layer on the nitride semiconductor active layer,vapor-phase-growing a second p-type nitride semiconductor layer havingan equivalent average Al composition to the first p-type nitridesemiconductor layer on the first p-type nitride semiconductor layer, andvapor-phase-growing a third p-type nitride semiconductor layer having asmaller average Al composition than the first p-type nitridesemiconductor layer on the second p-type nitride semiconductor layer, inwhich the second p-type nitride semiconductor layer and the third p-typenitride semiconductor layer are doped with a p-type impurity in lowerconcentrations than the first p-type nitride semiconductor layerrespectively.

Preferably in the method for producing a nitride semiconductorlight-emitting element according to the present invention, the vaporphase growth is interrupted after the step of vapor-phase-growing thesecond p-type nitride semiconductor layer and before the step ofvapor-phase-growing the third p-type nitride semiconductor layer.

Preferably in the method for producing a nitride semiconductorlight-emitting element according to the present invention, the pressureof the vapor phase is changed in the interruption of the vapor phasegrowth.

Preferably, the method for producing a nitride semiconductorlight-emitting element according to the present invention furtherincludes a step of vapor-phase-growing a fourth nitride semiconductorlayer doped with the p-type impurity in a higher concentration than thethird p-type nitride semiconductor layer on the third p-type nitridesemiconductor layer after the step of vapor-phase-growing the thirdp-type nitride semiconductor layer.

Preferably, the nitride semiconductor light-emitting element accordingto the present invention further includes a nitride semiconductor layercontaining Al between the nitride semiconductor active layer and thefirst p-type nitride semiconductor layer.

According to the present invention, a nitride semiconductorlight-emitting element excellent in characteristics and a method forproducing a nitride semiconductor light-emitting element can beprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a nitride semiconductorlight-emitting element according to an embodiment.

FIG. 2 is a schematic top plan view of the nitride semiconductorlight-emitting element according to the embodiment.

FIG. 3 is a diagram showing atomic concentration profiles of nitridesemiconductor light-emitting diode elements according to Example andcomparative example.

FIG. 4 is a schematic sectional view of a nitride semiconductorlight-emitting diode element according to Example 4.

FIG. 5 is a schematic top plan view of the nitride semiconductorlight-emitting diode element according to Example 4.

FIG. 6 is a diagram showing atomic concentration profiles of the nitridesemiconductor light-emitting diode element according to Example 4 and anitride semiconductor light-emitting diode element prepared by aproduction method similar to that in Example 1.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment of the present invention is now described. In the drawingsof the present invention, it is assumed that the same reference signsshow the same portions or corresponding portions.

FIG. 1 shows a schematic sectional view of a nitride semiconductorlight-emitting element according to this embodiment. In a nitridesemiconductor light-emitting element 100 according to this embodiment, anitride semiconductor buffer layer 102 made of AlN, a nitridesemiconductor intermediate layer 103 made of undoped GaN and an n-typenitride semiconductor underlayer 104 made of n-type GaN are stacked inthis order on a sapphire substrate 101 whose surface is irregularized,as shown in FIG. 1.

In this specification, a laminate of sapphire substrate 101, nitridesemiconductor buffer layer 102, nitride semiconductor intermediate layer103 and n-type nitride semiconductor underlayer 104 is employed as atemplate substrate. In place of the template substrate, a substrateobtained by forming nitride semiconductor layers on a substratedifferent from a nitride semiconductor or a substrate made of a nitridesemiconductor, for example, can also be employed.

An n-type nitride semiconductor contact layer 105 made of n-type GaN andan n-type nitride semiconductor superlattice layer 106 in which n-typeGaN layers and n-type InGaN layers are alternately stacked are stackedin this order on n-type nitride semiconductor underlayer 104 of thetemplate substrate.

N-type nitride semiconductor contact layer 105 is not restricted to thelayer made of n-type GaN, but a layer prepared by doping an n-typeimpurity into a nitride semiconductor layer made of a group III nitridesemiconductor expressed in a formula Al_(x1)Ga_(y1)In_(z1)N (0≦x1≦1,0≦y1≦1, 0≦z1≦1, and x1+y1+z1≠0) or the like can be stacked, for example.

In particular, a nitride semiconductor layer in which silicon is dopedinto a group III nitride semiconductor expressed in a formulaAl_(x2)Ga_(1-x2)N (0≦x2≦1, preferably 0≦x2≦0.5, and more preferably0≦x2≦0.1) as an n-type impurity is preferably employed as n-type nitridesemiconductor contact layer 105. In this case, there is a tendency thatthe doping concentration of the n-type impurity can be increased whileensuring excellent crystallinity of n-type nitride semiconductor contactlayer 105.

The doping concentration of the n-type impurity in n-type nitridesemiconductor contact layer 105 is preferably set to at least 5×10¹⁷atoms/cm³ and not more than 5×10¹⁹ atoms/cm³. In a case where the dopingconcentration of the n-type impurity in n-type nitride semiconductorcontact layer 105 is at least 5×10¹⁷ atoms/cm³ and not more than 5×10¹⁹atoms/cm³, there is a tendency that excellent ohmic contact with ann-side pad electrode 117 described later can be ensured while there is atendency that occurrence of cracking in n-type nitride semiconductorcontact layer 105 can be suppressed, and there is also a tendency thatexcellent crystallinity of n-type nitride semiconductor contact layer105 can be ensured.

The total of the thicknesses of nitride semiconductor intermediate layer103 made of undoped GaN, n-type nitride semiconductor underlayer 104made of n-type GaN and n-type nitride semiconductor contact layer 105made of n-type GaN is preferably at least 4 μm and not more than 20 m,more preferably at least 4 μm and not more than 15 μm, and furtherpreferably at least 6 μm and not more than 15 μm, in view of ensuringexcellent crystallinity of these layers. In a case where the total ofthe thicknesses of these layers is less than 4 μm, there is anapprehension that the crystallinity of these layers deteriorates or pitsare formed on the surfaces of these layers. In a case where the total ofthe thicknesses of these layers exceeds 20 μm, there is an apprehensionthat warpage of sapphire substrate 101 so enlarges that various problemsarise in a production process. In a case where the total of thethicknesses of these layers is at least 4 μm and not more than 15 μm,particularly in a case where the same is at least 6 μm and not more than15 μm, there is a tendency that the crystallinity of these layers can berendered excellent. In the total of the thicknesses of these layers, theupper limit of the thickness of n-type nitride semiconductor contactlayer 105 is not particularly restricted.

While the thickness of n-type nitride semiconductor superlattice layer106 is not particularly restricted, the same is preferably at least0.005 μm and not more than 0.5 μm, and more preferably at least 0.01 μmand not more than 0.1 μm. In a case where the thickness of n-typenitride semiconductor superlattice layer 106 is at least 0.005 μm andnot more than 0.5 μm, particularly in a case where the same is at least0.01 μm and not more than 0.1 μm, there are tendencies that n-typenitride semiconductor superlattice layer 106 reduces the number ofcrystal defects extending from n-type nitride semiconductor contactlayer 105 which is the underlayer and that new crystal defects hardlyarise in n-type nitride semiconductor superlattice layer 106.

As to a doping profile of the n-type impurity in n-type nitridesemiconductor superlattice layer 106, the n-type impurity is doped onlyinto the GaN layers, and a set impurity concentration thereof ispreferably at least 5×10¹⁶ atoms/cm³ and not more than 1×10²⁰ atoms/cm³,in view of ensuring excellent crystallinity and reducing operatingvoltage for the element. The structure of n-type nitride semiconductorsuperlattice layer 106 is not restricted to any of the aforementionedstructures.

An average n-type impurity concentration in n-type nitride semiconductorsuperlattice layer 106 is preferably at least 1×10¹⁸ atoms/cm³, morepreferably at least 3×10¹⁸ atoms/cm³, and further preferably 5×10¹⁸atoms/cm³. In a case where the n-type impurity concentration in thelayers of n-type nitride semiconductor superlattice layer 106 doped withthe n-type impurity is the aforementioned value, a depletion layer ofp-n junction hardly spreads, whereby there are tendencies thatelectrostatic capacity of nitride semiconductor light-emitting element100 increases, that breakage (electrostatic discharge damage) resultingfrom electrostatic discharge (Electrostatic Discharge) hardly takesplace, and that an electrostatic discharge threshold improves. Theaverage n-type impurity concentration is a value obtained by dividingthe total atomic weight of the n-type impurity in n-type nitridesemiconductor superlattice layer 106 by the volume of n-type nitridesemiconductor superlattice layer 106.

A nitride semiconductor active layer 107, in which n-type nitridesemiconductor barrier layers made of n-type GaN and undoped InGaNnitride semiconductor quantum well layers are alternately stacked inthis order from the side of n-type nitride semiconductor superlatticelayer 106, prepared by stacking a nitride semiconductor barrier layermade of undoped GaN as the uppermost layer is stacked on n-type nitridesemiconductor superlattice layer 106. Layers holding the quantum welllayers therebetween are formed by the barrier layers, and hence thenumber of the barrier layers is the number of the quantum well layers+1.Thus, combined positions of holes and electrons can be made to exist inmultiple quantum wells (MQW) by doping an n-type impurity into thebarrier layers other than the uppermost layer of n-type semiconductoractive layer 107 and rendering the uppermost barrier layer undoped,whereby carrier injection efficiency increases, and a nitridesemiconductor light-emitting element having such high luminousefficiency that electrons do not overflow also in a case of injectingcurrent of particularly high current density.

A substance having an MQW structure prepared by employing nitridesemiconductor layers made of a group III nitride semiconductor expressedin a formula Ga_(1-z4)In_(z4)N (0<z4<0.4) as quantum well layers,employing nitride semiconductor layers made of a nitride semiconductorexpressed in a formula Al_(x3)Ga_(y3)In_(z3)N (0≦x3≦1, 0≦y3≦1, 0≦z3≦1,and x3+y3+z3≠0) having a larger band gap than these quantum well layersas barrier layers and alternately stacking the quantum well layers andthe barrier layers one by one, for example, can be employed as nitridesemiconductor active layer 107, in place of the above.

In a case where nitride semiconductor active layer 107 consists of amultiple quantum well (MQW) structure having nitride semiconductorlayers made of the group III nitride semiconductor expressed in theformula Ga_(1-z4)In_(z4)N (0<z4<0.4) as quantum well layers, forexample, the In composition in nitride semiconductor active layer 107and the thickness thereof can be controlled to obtain a desired emissionwavelength.

A first p-type nitride semiconductor layer 108 made of p-type AlGaN, asecond p-type nitride semiconductor 109 made of undoped AlGaN, a thirdp-type nitride semiconductor layer 110 made of undoped GaN and a fourthp-type nitride semiconductor layer 111 made of p⁺ GaN doped with ap-type impurity in a high concentration are stacked in this order onnitride semiconductor active layer 107.

As first p-type nitride semiconductor layer 108, a layer prepared bydoping a p-type impurity such as Mg, for example, into a nitridesemiconductor layer made of an Al-containing group III nitridesemiconductor expressed in a formula Al_(x5)Ga_(y5)In_(z5)N (0<x5≦1,0≦y5≦1, and 0≦z5≦1), for example, in place of p-type AlGaN can bestacked.

As second p-type nitride semiconductor layer 109, a layer prepared bydoping a p-type impurity such as Mg, for example, into a nitridesemiconductor layer made of an Al-containing group III nitridesemiconductor expressed in a formula Al_(x6)Ga_(y6)In_(z6)N (0<x6≦1,0≦y6≦1, and 0≦z6≦1), for example, in place of undoped AlGaN can bestacked.

The band gaps of the respective ones of first p-type nitridesemiconductor layer 108 and second p-type nitride semiconductor layer109 are preferably rendered larger than the band gap of nitridesemiconductor active layer 107, in order to suppress overflowing ofelectrons. Therefore, nitride semiconductor layers containing Al can beemployed for first p-type nitride semiconductor layer 108 and secondp-type nitride semiconductor layer 109 respectively.

The average Al composition (x5) in first p-type nitride semiconductorlayer 108 and the average Al composition (x6) in second p-type nitridesemiconductor layer 109 are equivalent to each other. The average Alcomposition (x5) in first p-type nitride semiconductor layer 108 and theaverage Al composition (x6) in second p-type nitride semiconductor layer109 are so rendered equivalent to each other that dispersion inincorporability of the p-type impurity such as Mg can be suppressed,whereby dispersion in in-plane distribution of the p-type impurity orbetween wafers can be suppressed.

While the average Al composition (x5) in first p-type nitridesemiconductor layer 108 and the average Al composition (x6) in secondp-type nitride semiconductor layer 109 reach the Al compositions x5 andx6 respectively in a case where the Al compositions in the respectivelayers are uniform, the average compositions Al_(x5)Ga_(y5)In_(z5)N(0<x5≦1, 0≦y5≦1, and 0≦z5≦1) and Al_(x6)Ga_(y6)In_(z6)N (0<x6≦1, 0≦y6≦1,and 0≦z6≦1) can be obtained by obtaining the ratios of the total atomicweight of Al, the total atomic weight of Ga, the total atomic weight ofIn and the total atomic weight of N with respect to the volumes of therespective layers and dividing these by the ratio of the total atomicweight of N respectively in a case where the same are nonuniform alongthe layer thickness direction.

In order that the average Al composition (x5) in first p-type nitridesemiconductor layer 108 and the average Al composition (x6) in secondp-type nitride semiconductor layer 109 are equivalent to each other, theabsolute value of the difference between the average Al composition (x5)in first p-type nitride semiconductor layer 108 and the average Alcomposition (x6) in second p-type nitride semiconductor layer 109 maysimply be not more than 0.02.

The p-type impurity concentration in second p-type nitride semiconductorlayer 109 is lower than the p-type impurity concentration in firstp-type nitride semiconductor layer 108. In a case where the p-typeimpurity concentration in first p-type nitride semiconductor layer 108on the side close to nitride semiconductor active layer 107 is low ascompared with the p-type impurity concentration in second p-type nitridesemiconductor layer 109, barriers of holes in nitride semiconductoractive layer 107 so heighten that the hole injection efficiencydeteriorates and the luminous efficiency of the element lowers.

In a case where the p-type impurity concentration in first p-typenitride semiconductor layer 108 on the side close to nitridesemiconductor active layer 107 is low and the p-type impurityconcentration in second p-type nitride semiconductor layer 109 is high,a depletion layer spreads up to second p-type nitride semiconductorlayer 109 in a case where high reverse voltage of static electricity orthe like is applied. At this time, electrostatic discharge damage takesplace in a case where a portion having a low electrostatic dischargethreshold is present even in one spot of the spreading depletion layer,and hence nitride semiconductor light-emitting element 100 enters astructure having a weak electrostatic discharge threshold.

In a case where the p-type impurity concentration in first p-typenitride semiconductor layer 108 is high and the n-type impurityconcentration in n-type nitride semiconductor superlattice layer 106 ishigh, on the other hand, a depletion layer hardly spreads outside firstp-type nitride semiconductor layer 108 and n-type nitride semiconductorsuperlattice layer 106, and hence the electrostatic discharge thresholdheightens.

In a case of rendering the p-type impurity concentration in secondp-type nitride semiconductor layer 109 identical to the p-type impurityconcentration in first p-type nitride semiconductor layer 108 or inexcess thereof, the p-type impurity such as Mg so deposits in thesubsequent stacking of third p-type nitride semiconductor layer 110 thata layer whose p-type impurity concentration locally heightens is formedbetween second p-type nitride semiconductor layer 109 and third p-typenitride semiconductor layer 110. Such a layer lowers the hole injectionefficiency into nitride semiconductor active layer 107 and lowers theluminous efficiency of the element, and hence the p-type impurityconcentration in second p-type nitride semiconductor layer 109 isrendered lower than the p-type impurity concentration in first p-typenitride semiconductor layer 108.

The p-type impurity concentrations (atomic concentrations) in the layerscan be obtained by SIMS (Secondary Ion Mass Spectrometry).

First p-type nitride semiconductor layer 108 and second p-type nitridesemiconductor layer 109 may have superlattice structures obtained byalternately stacking AlGaN layers and GaN layers (may be InGaN layers),for example, respectively. In a case where first p-type nitridesemiconductor layer 108 and/or second p-type nitride semiconductor layer109 has a superlattice structure, it is assumed that the average Alcomposition is the average composition of Al in the superlatticestructure.

While the total of the thicknesses of first p-type nitride semiconductorlayer 108 and second p-type nitride semiconductor layer 109 is notparticularly restricted, the same is preferably at least 0.005 μm andnot more than 0.4 μm, and more preferably at least 0.001 μm and not morethan 0.1 μm. In a case where the total of the thicknesses of theselayers is at least 0.005 μm and not more than 0.4 μm, particularly in acase where the same is at least 0.001 μm and not more than 0.1 μm, thereis a tendency that diffusion of the p-type impurity into nitridesemiconductor active layer 107 which is a low temperature growth layeror alteration resulting from a heat history can be suppressed byminimizing growth times of the p-type nitride semiconductor layers whichare high temperature growth layers while maintaining functions as thep-type nitride semiconductor layers.

As third p-type nitride semiconductor layer 110, a layer prepared bydoping a p-type impurity such as Mg, for example, into a nitridesemiconductor layer made of a group III nitride semiconductor expressedin a formula Al_(x7)Ga_(y7)In_(z7)N (0≦x7≦1, 0≦y7≦1, 0≦z7≦1, andx7+y7+z7≠0), for example, in place of undoped GaN can be stacked.

The average Al composition in third p-type nitride semiconductor layer110 is preferably smaller than the average Al composition in the firstp-type nitride semiconductor layer. In a case where the average Alcomposition in third p-type nitride semiconductor layer 110 is smallerthan the average Al composition in the first p-type nitridesemiconductor layer, the average Al composition is so small thatcharacteristics as the p-type nitride semiconductor layer improve. Almay not be contained in third p-type nitride semiconductor layer 110.

The band gap of third p-type nitride semiconductor layer 110 is renderedsmaller than the band gap of second p-type nitride semiconductor layer109. While the function as the p-type nitride semiconductor layer isimportant for third p-type nitride semiconductor layer 110 and hencelargeness of the band gap is not particularly required thereto, the bandgap of second p-type nitride semiconductor layer 109 must be enlarged inorder to prevent overflowing of electrons from nitride semiconductoractive layer 107 to the p-side.

The p-type impurity concentration in third p-type nitride semiconductorlayer 110 is rendered lower than the p-type impurity concentration infirst p-type nitride semiconductor layer 108. In the case where thep-type impurity concentration in third p-type nitride semiconductorlayer 110 is lower than the p-type impurity concentration in firstp-type nitride semiconductor layer 108, there are such tendencies that aportion whose p-type impurity concentration heightens is hardly formedbetween second p-type nitride semiconductor layer 109 and third p-typenitride semiconductor layer 110 and that diffusion of the p-typeimpurity into nitride semiconductor active layer 107 hardly takes place.

As fourth p-type nitride semiconductor layer 111, a layer prepared bydoping a p-type impurity such as Mg, for example, into a nitridesemiconductor layer made of a group III nitride semiconductor expressedin a formula Al_(x8)Ga_(y8)In_(z8)N (0≦x8≦1, 0≦y8≦1, 0≦z8≦1, andx8+y8+z8≠0), for example, in place of p⁺ GaN can be stacked.

The band gap of fourth p-type nitride semiconductor layer 111 ispreferably smaller than the band gap of second p-type nitridesemiconductor layer 109. This is because the function as the p-typenitride semiconductor layer is important for fourth p-type nitridesemiconductor layer 111 and hence largeness of the band gap is notparticularly required thereto, while the band gap of first p-typenitride semiconductor layer 108 must preferably be enlarged in order toprevent overflowing of electrons from nitride semiconductor active layer107 to the p-side.

The p-type impurity concentration in fourth p-type nitride semiconductorlayer 111 is preferably higher than the p-type impurity concentration inthird p-type nitride semiconductor layer 110. In a case where the p-typeimpurity concentration in fourth p-type nitride semiconductor layer 111is higher than the p-type impurity concentration in third p-type nitridesemiconductor layer 110, there is a tendency that the total quantity ofthe diffusing p-type impurity can be suppressed while keeping contactresistance between fourth p-type nitride semiconductor layer 111 and atransparent electrode 115 described later sufficiently low.

While the thickness of fourth p-type nitride semiconductor layer 111 isnot particularly restricted, the same is preferably at least 0.01 μm andnot more than 0.5 μm, and more preferably at least 0.05 μm and not morethan 0.2 μm. In a case where the thickness of fourth p-type nitridesemiconductor layer 111 is at least 0.01 μm and not more than 0.5 μm,particularly in a case where the same is at least 0.05 μm and not morethan 0.2 μm, there is a tendency that an emission output of the nitridesemiconductor light-emitting element improves.

Transparent electrode 115 is formed on part of the surface of fourthp-type nitride semiconductor layer 111. A p-side pad electrode 116 isformed on part of the surface of transparent electrode 115 to beelectrically connected thereto, and an n-side pad electrode 117 isformed on part of the surface of n-type nitride semiconductor contactlayer 105 to be electrically connected thereto.

FIG. 2 shows a schematic top plan view of the nitride semiconductorlight emitting element according to this embodiment. As shown in FIG. 2,p-side pad electrode 116 is formed within the surface of transparentelectrode 116.

As the material for transparent electrode 115, ITO (Indium Tin Oxide),tin oxide, indium oxide, zinc oxide, gallium oxide, IZO (Indium ZincOxide), AZO (Aluminum Zinc Oxide) or GZO (gallium Zinc Oxide), forexample, can be employed.

The thickness of transparent electrode 115 is preferably set to at least10 nm and not more than 1000 nm, and more preferably set to at least 50nm and not more than 500 nm. In a case where the thickness oftransparent electrode 115 is at least 10 nm and not more than 1000 nm,particularly in a case where the same is at least 50 nm and not morethan 500 nm, there are tendencies that balance between an effect ofprompting diffusion of current by reducing specific resistance oftransparent electrode 115 and an effect of improving light extractionefficiency by ensuring light transmittance of transparent electrode 115is excellent and that the light output can be maximized.

As p-side pad electrode 116 and n-side pad electrode 117, laminates ofNi layers and Au layers, laminates of Ti layers and Al layers, laminatesof Ti—Al alloy layers, Hf layers and Al layers or Hf—Al alloy layers canbe employed, for example.

As p-side pad electrode 116 and n-side pad electrode 117, transparentconductive films of ITO or the like may also be employed, and a flipchip type light-emitting element can be obtained by employing metals ofhigh reflectance.

An example of a method for producing the nitride semiconductorlight-emitting element according to this embodiment is now described.First, the template substrate is formed by vapor-phase-growing nitridesemiconductor buffer layer 102 made of AlN, for example, nitridesemiconductor intermediate layer 103 made of undoped GaN, for example,and n-type nitride semiconductor underlayer 104 made of n-type GaN, forexample, on sapphire substrate 101 in this order by MOCVD (Metal OrganicChemical Vapor Deposition), for example.

Then, n-type nitride semiconductor contact layer 105 made of n-type GaN,for example, and n-type nitride semiconductor superlattice layer 106 inwhich n-type GaN layers and undoped InGaN layers are alternatelystacked, for example, are vapor-phase-grown on n-type nitridesemiconductor underlayer 104 of the template substrate in this order byMOCVD, for example.

Then, nitride semiconductor active layer 107, in which n-type nitridesemiconductor barrier layers made of n-type GaN and undoped InGaNnitride semiconductor quantum well layers are alternately stacked inthis order from the side of n-type nitride semiconductor superlatticelayer 106, prepared by stacking a nitride semiconductor barrier layermade of undoped GaN as the uppermost layer, for example, isvapor-phase-grown on n-type nitride semiconductor superlattice layer 106by MOCVD, for example.

In the case where nitride semiconductor active layer 107 consists of themultiple quantum well (MQW) structure having the nitride semiconductorlayers made of the group III nitride semiconductor expressed in theformula Ga_(1-z4)In_(z4)N (0<z4<0.4) as the quantum well layers, forexample, there is an apprehension that crystallinity deteriorates if thetemperature of the template substrate at the time of the vapor phasegrowth of nitride semiconductor active layer 107 is low, while there isan apprehension that sublimation of InN is so remarkable thatincorporation efficiency of In into a solid phase lowers and the Incomposition fluctuates if the temperature of the template substrate atthe time of the vapor phase growth of nitride semiconductor active layer107 is high. In the case where nitride semiconductor active layer 107consists of the multiple quantum well (MQW) structure having the nitridesemiconductor layers made of the group III nitride semiconductorexpressed in the formula Ga_(1-z4)In_(z4)N (0<z4<0.4) as the quantumwell layers, for example, therefore, the temperature of the templatesubstrate in the formation of nitride semiconductor active layer 107 ispreferably at least 700° C. and not more than 900° C.

Then, Al-containing first p-type nitride semiconductor layer 108 ofp-type AlGaN or the like, for example, is vapor-phase-grown on nitridesemiconductor active layer 107 by MOCVD, for example. In a case wherefirst p-type nitride semiconductor layer 108 is made of p-type AlGaN, auniform film of high quality can be obtained by growing first p-typenitride semiconductor layer 108 with high pressure while growing thesame without fluctuating the quantity of group III source gas and thequantity of group V source gas.

Then, Al-containing second p-type nitride semiconductor layer 109 ofundoped AlGaN, for example, is vapor-phase-grown on first p-type nitridesemiconductor layer 108 by MOCVD, for example. In this specification,undoped denotes that no impurity is intentionally introduced in vaporphase growth, and an impurity may be contained even in an undoped layerdue to a reason such as diffusion. In a case where second p-type nitridesemiconductor layer 109 is made of p-type AlGaN, a uniform film of highquality can be obtained by growing second p-type nitride semiconductorlayer 109 with low pressure while growing the same without fluctuatingthe quantity of group III source gas and the quantity of group V sourcegas.

As hereinabove described, second p-type nitride semiconductor layer 109is so formed that the average Al composition is lower than that in firstp-type nitride semiconductor layer 108, and formed by doping the p-typeimpurity thereinto so that the p-type impurity concentration is lowerthan that in first p-type nitride semiconductor layer 108.

Then, second p-type nitride semiconductor layer 109 consisting of thirdp-type nitride semiconductor layer 110 made of undoped GaN, for example,is vapor-phase-grown on second p-type nitride semiconductor layer 109 byMOCVD, for example. Third p-type nitride semiconductor layer 110 ispreferably so formed that the average Al composition is lower than thatin first p-type nitride semiconductor layer 108. Further, third p-typenitride semiconductor layer 110 is preferably formed by doping thep-type impurity thereinto so that the p-type impurity concentration islower than that in first p-type nitride semiconductor layer 108.

The vapor phase growth is preferably interrupted aftervapor-phase-growing second p-type nitride semiconductor layer 109 andbefore vapor-phase-growing third p-type nitride semiconductor layer 110.In this case, there is a tendency that the pressure can be changed topressure of the vapor phase suitable to the growth rate of third p-typenitride semiconductor layer 110 during the interruption of the vaporphase growth.

Then, fourth p-type nitride semiconductor layer 111 made of p⁺ GaN, forexample, is vapor-phase-grown on third p-type nitride semiconductorlayer 110 by MOCVD, for example. Fourth p-type nitride semiconductorlayer 111 is preferably formed by doping the p-type impurity thereintoso that the p-type impurity concentration is higher than that in thirdp-type nitride semiconductor layer 110.

Then, transparent electrode 115 made of ITO, for example, is formed onpart of the surface of fourth p-type nitride semiconductor layer 111 bysputtering or vacuum evaporation, for example.

Then, part of the surface of n-type nitride semiconductor contact layer105 is exposed by removing part of a wafer after the formation oftransparent electrode 115 by etching.

Then, p-side pad electrode 116 is formed on part of the surface oftransparent electrode 115, while n-side pad electrode 117 is formed onpart of the surface of n-type nitride semiconductor contact layer 105.Thereafter the nitride semiconductor light-emitting element (nitridesemiconductor light-emitting diode element) according to this embodimentis obtained by dividing the wafer into individual elements by scribingor the like.

In the nitride semiconductor light-emitting element according to thisembodiment, it is possible to compatibly attain such generallyconflictive structures that the p-type impurity can be inhibited fromdiffusing into nitride semiconductor active layer 107 and p-type nitridesemiconductor layers of high p-type impurity concentrations can be setin the vicinity of nitride semiconductor active layer 107 also afterthrough heat treatment steps in the production process for the nitridesemiconductor light-emitting element. Thus, a nitride semiconductorlight-emitting element of high luminous efficiency can be implemented.

As a nitride semiconductor light-emitting diode element according toExample 1, an MN buffer layer 102, an undoped GaN layer 103, an n-typeGaN underlayer 104 (n-type impurity (Si) concentration: 6×10¹⁸atoms/cm³), an n-type GaN contact layer 105 (n-type impurity (Si)concentration: 6×10¹⁸ atoms/cm³, thickness: 1.5 μm), an n-typesuperlattice layer 106 formed by alternately stacking Si-doped n-typeGaN layers (n-type impurity (Si) concentration: 5×10¹⁸ atoms/cm³,thickness: 1.75 nm) and Si-doped InGaN layers (n-type impurity (Si)concentration: 5×10¹⁸ atoms/cm³, thickness: 1.75 nm) by 20 cycles, anitride semiconductor active layer 107 formed by stacking an undoped GaNbarrier layer after alternately stacking Si-doped n-type GaN barrierlayers (n-type impurity (Si) concentration: 4×10¹⁷ atoms/cm³, thickness:6.5 nm) and undoped InGaN quantum well layers (thickness: 3.5 nm) by sixcycles, a first p-type nitride semiconductor layer 108 (p-type impurity(Mg) concentration: 2×10¹⁹ atoms/cm³, thickness: 11.75 nm) made ofMg-doped p-type AlGaN, a second p-type nitride semiconductor layer 109(p-type impurity (Mg) concentration: 1×10¹⁹ atoms/cm³, thickness: 3.75nm) made of undoped AlGaN, a third p-type nitride semiconductor layer110 (p-type impurity (Mg) concentration: 1×10¹⁹ atoms/cm³, thickness: 60nm) made of undoped GaN and a fourth p-type nitride semiconductor layer111 (p-type impurity (Mg) concentration: 3×10¹⁹ atoms/cm³, thickness: 20nm) made of p⁺ GaN are stacked in this order on a sapphire substrate 101whose surface is irregularized, as shown in FIGS. 1 and 2.

A transparent electrode 115 made of ITO is formed on part of the surfaceof fourth p-type nitride semiconductor layer 107, a p-side pad electrode116 consisting of a laminate of an Ni layer and an Au layer from theside close to transparent electrode 115 is formed on part of the surfaceof transparent electrode 115, and an n-side pad electrode 117 consistingof a laminate of an Ni layer and an Au layer is formed on part of thesurface of n-type GaN contact layer 105.

The nitride semiconductor light-emitting diode element according toExample 1 having the aforementioned structure was prepared as follows:

First, a template substrate was prepared by forming AlN buffer layer 102on sapphire substrate 101 whose surface was irregularized by sputteringor MOCVD, and stacking undoped GaN layer 103 and Si-doped n-type GaNunderlayer 104 (n-type impurity (Si) concentration: 6×10¹⁸ atoms/cm³) inthis order on MN buffer layer 102. The Si doping concentration in n-typeGaN underlayer was set to 6×10¹⁸ atoms/cm³.

Then, the template substrate was set in an MOCVD apparatus which was avapor phase growth apparatus, and the pressure of a vapor phase in theMOCVD apparatus was controlled to 2×10² Pa. Then, the temperature of thetemplate substrate was raised to 1195° C. while feeding H₂ gas ascarrier gas and NH₃ gas as a group V raw material into the MOCVDapparatus, to vapor-phase-grow n-type GaN contact layer 105 (n-typeimpurity (Si) concentration: 6×10¹⁸ atoms/cm³) on n-type GaN underlayer104 of the template substrate into the thickness of 1.5 μm. SiH₄ gas wasemployed for an Si dopant raw material employed for the vapor phasegrowth of n-type GaN contact layer 105, and the Si doping concentrationwas set to 6×10¹⁸ atoms/cm³.

Then, after the vapor phase growth of n-type GaN contact layer 105, thecarrier gas was switched from the H₂ gas to N₂ gas, and the temperatureof the template substrate was lowered to 870° C.

Then, n-type superlattice layer 106 having a total thickness of 70 nmwas stacked by alternately stacking 20 pairs of the Si-doped n-type GaNlayers of 1.75 nm in thickness and the n-type InGaN layers of 1.75 nm inthickness. SiH₄ gas was employed for the Si dopant raw material. N-typesuperlattice layer 106 is rendered highly doped in this manner so that adepletion layer does not spread up to n-type GaN contact layer 105 belown-type superlattice layer 106 or n-type GaN underlayer 104 of thetemplate substrate even if high reverse voltage of static electricity orthe like is applied, whereby an electrostatic discharge thresholdheightens. This is because, while a crystal deteriorates in crystalquality as approaching the substrate and there exists a portion weakagainst electrostatic electricity or the like, the same is notelectrostatically broken if the depletion layer does not spread thereto.

After vapor-phase-growing n-type superlattice layer 106, the temperatureof the template substrate was lowered to 850° C. Then, n-typesemiconductor active layer 107 having an MQW structure whose totalthickness was 66.5 nm was formed by stacking six pairs of Si-dopedn-type GaN barrier layers of 6.5 nm in thickness and undoped InGaNquantum well layers of 3.5 nm in thickness, thereafter raising thetemperature of the template substrate to 860° C. and stacking an undopedGaN barrier layer of 6.5 nm in thickness. SiH₄ gas was employed for theSi dopant raw material employed for the vapor phase growth of theSi-doped n-type GaN barrier layers.

Thus, combined positions of holes and electrons can be set in the MQW ofnitride semiconductor active layer 107 by doping Si into the first sixlayers included in the barrier layers constituting nitride-semiconductoractive layer 107 and rendering the last barrier layer undoped, and anitride semiconductor light-emitting element having high luminousefficiency with no overflowing of electrons also in a case of injectinghigh current is obtained.

While the temperature of the template substrate is raised in order togrow p-type layers described later after vapor-phase-growing nitridesemiconductor active layer 107, the undoped GaN barrier layer formingthe uppermost layer of nitride semiconductor active layer 107 is easilydamaged. However, the undoped GaN barrier layer forming the uppermostlayer is rendered undoped and the undoped GaN barrier layer forming theuppermost layer is vapor-phase-grown after raising the temperature ofthe template substrate to 860°, whereby crystal quality of the undopedGaN barrier layer forming the uppermost layer improves, and damageresulting from the temperature rise of the temperate substrate can beprevented.

Then, the pressure of the vapor phase in the MOCVD apparatus was loweredto 1×10⁴ Pa, while the temperature of the template substrate was raisedto 1110° C. Dispersion of Al compositions in AlGaN layers describedlater between wafers and dispersion of Al compositions in the wafers inthe MOCVD apparatus capable of growing a large number of wafers can besuppressed and the yield of elements can be improved by lowering thepressure of the vapor phase in the MOCVD apparatus to 1×10⁴ Pa.

Then, the carrier gas supplied into the MOCVD apparatus was switchedfrom the N₂ gas to H₂ gas, for vapor-phase-growing first p-type nitridesemiconductor layer 108 (p type impurity (Mg) concentration: 2×10¹⁹atoms/cm³) made of Mg-doped p-type Al_(0.17)Ga_(0.83)N having athickness of 11.25 nm while vapor-phase-growing second p-type nitridesemiconductor layer 109 (p-type impurity (Mg) concentration: 1×10¹⁹atoms/cm³) consisting of an undoped Al_(0.17)Ga_(0.83)N layer having thethickness of 3.75 nm.

The average Al composition in first p-type nitride semiconductor layer108 was set to 17%, while Cp₂Mg (biscyclopentadienyl magnesium) gas wasemployed as an Mg dopant raw material for first p-type nitridesemiconductor layer 108 and the Mg doping concentration was set to2×10¹⁹ atoms/cm³.

The average Al composition in second p-type nitride semiconductor layer109 was set to 17%, and no Cp₂Mg gas was fed during the vapor phasegrowth of second p-type nitride semiconductor layer 109. However, Mg wasincorporated into actually grown second p-type semiconductor layer 109made of undoped AlGaN due to diffusion, and the Mg atomic concentrationin second p-type nitride semiconductor layer 109 was 1×10¹⁹ atoms/cm³.

Then, the vapor phase growth was temporarily interrupted and thepressure of the vapor phase in the MOCVD apparatus was raised to 2×10⁴Pa. Thereafter vapor phase growth was started, to vapor-phase-grow thirdp-type nitride semiconductor layer 110 (p-type impurity (Mg)concentration: 1×10¹⁹ atoms/cm³) made of undoped GaN having thethickness of 60 nm and to vapor-phase-grow fourth p-type nitridesemiconductor layer 111 (p-type impurity (Mg) concentration: 3×10¹⁹atoms/cm³) made of p⁺ GaN having a thickness of 30 nm. The reason forreturning the pressure of the vapor phase in the MOCVD apparatus to2×10⁴ Pa is that the rate of activation of Mg improves and operatingvoltage for the element can be lowered.

No Cp₂Mg gas was fed during the vapor phase growth of third p-typenitride semiconductor layer 110. However, Mg was incorporated intoactually grown third p-type nitride semiconductor layer 110 made ofundoped GaN due to diffusion, and the Mg atomic concentration in thirdp-type nitride semiconductor layer 110 was 1×10¹⁹ atoms/cm³.

The Mg doping concentration in fourth p-type nitride semiconductor layer111 made of p⁺ GaN was set to 3×10¹⁹ atoms/cm³. The Mg dopingconcentration in fourth p-type nitride semiconductor layer 111 is so setto the highly doped level of 3×10¹⁹ atoms/cm³ that contact resistancebetween fourth p-type nitride semiconductor layer 111 and transparentelectrode 115 made of ITO described later, whereby driving voltage forthe element can be reduced.

Then, annealing at 950° C. was performed in an N₂ gas atmosphere, inorder to prompt control of first p-type nitride semiconductor layer 108,second p-type nitride semiconductor layer 109, third p-type nitridesemiconductor layer 110 and fourth p-type nitride semiconductor layer111 to the p types.

Then, transparent electrode 115 made of ITO was formed on part of thesurface of fourth p-type nitride semiconductor layer 111 by sputtering.Then, annealing at 500° C. was performed on the wafer after theformation of transparent electrode 115 in an atmosphere containingoxygen, in order to improve characteristics of ITO.

Thereafter part of the surface of n-type GaN contact layer 105 wasexposed by removing part of the wafer after the formation of transparentelectrode 115 by etching. Then, p-side pad electrode 116 and n-side padelectrode 117 consisting of the Ni layers and the Au layers weresimultaneously formed on the surface of transparent electrode 115 and onthe surface of n-type GaN contact layer 105 respectively by vacuumevaporation.

Thereafter a protective film (not shown) covering the whole other thanp-side pad electrode 116 and n-side pad electrode 117 was formed, andheat treatment at 300° C. was performed in the state forming theprotective film, in order to reduce contact resistance between p-sidepad electrode 116 and transparent electrode 115 and contact resistancebetween the n-side pad electrode 113 and n-type GaN contact layer 105respectively. Thereafter the nitride semiconductor light-emitting diodeelement according to Example 1 was obtained by dividing the wafer intoindividual elements by scribing and application of bending stress.

As a result of testing 10 samples of the nitride semiconductorlight-emitting diode element according to Example 1 obtained in theaforementioned manner in chip states (before packaging by resinsealing), an emission output was 37 mW on the average with operatingcurrent of 30 mA. Electrostatic discharge thresholds of the samples ofthe nitride semiconductor light-emitting diode element according toExample 1 were excellent in all of the 10 samples as a result of testingthe same by an HBM (Human Body Model) method with application of 1500 V,and hence the same were at least 1500 V. Therefore, the nitridesemiconductor light-emitting diode element according to Example 1 wasexcellent in both emission characteristics and electrostatic dischargethreshold.

Comparative Example

A nitride semiconductor light-emitting diode element according tocomparative example was prepared as follows: First, elements up to anitride semiconductor active layer 107 were prepared similarly toExample 1. Then, carrier gas was switched from N₂ gas to H₂ gas, forvapor-phase-growing a p-type AlGaN layer on nitride semiconductor activelayer 107 into a thickness of 15 nm (growth conditions were identical tothose for first p-type nitride semiconductor layer 108).

Then, after raising the pressure of a vapor phase in an MOCVD apparatusup to 2×10⁴ Pa, a p-type GaN layer was vapor-phase-grown into athickness of 60 nm (growth conditions were identical to those for thirdp-type nitride semiconductor layer 110). Then, a p⁺ GaN layer wasvapor-phase-grown into a thickness of 30 nm (growth conditions wereidentical to those for fourth p-type nitride semiconductor layer 111).Thereafter the nitride semiconductor light-emitting diode elementaccording to comparative example was prepared similarly to Example 1.

In the nitride semiconductor light-emitting diode element according tocomparative example obtained in the aforementioned manner, an emissionoutput was 33 mW with operating current of 30 mA, and the emissionoutput was lower by about 10 percent as compared with the nitridesemiconductor light-emitting diode element according to Example 1.

FIG. 3 shows atomic concentration profiles of the nitride semiconductorlight-emitting diode element according to Example 1 and the nitridesemiconductor light-emitting diode element according to comparativeexample by SIMS (Secondary Ion Mass Spectrometry). The axis of abscissasin FIG. 3 shows depths (depth (a.u.)), and the axis of ordinates showsatomic concentrations (concentration (a.u.)). Referring to FIG. 3, 1-Mgdenotes the Mg atomic concentration in Example 1, 0-Mg denotes the Mgatomic concentration in comparative example, Al denotes relative Alatomic concentrations (substantially coinciding in Example 1 andcomparative example), and In denotes relative In atomic concentrations(substantially coinciding in Example 1 and comparative example).

The first feature in FIG. 3 resides in Mg atomic concentrations inuppermost quantum well layers in nitride semiconductor active layers107. A vertical line is drawn on a portion corresponding to theuppermost layers of the quantum well layers of six cycles in FIG. 3, andit is assumed that Y0 and Y1 represent the Mg atomic concentration incomparative example and the Mg concentration in Example 1 on the portionrespectively. Comparing both of the Mg atomic concentration of Y0 andthe Mg atomic concentration of Y1 with each other, it is understood thatthe nitride semiconductor light-emitting diode element according toExample 1 was capable of reducing the Mg concentration in the uppermostlayer of the quantum well layers to about half as compared with thenitride semiconductor light-emitting diode element according tocomparative example.

The second feature in FIG. 3 resides in the presence of a point X wherethe Mg concentration heightens between the p-type AlGaN layer and thep-type GaN layer in the Mg atomic concentration profile of comparativeexample. The inventors have noted that a portion of a high concentrationis present on this interface in comparative example, and found that itis effective to perform vapor phase growth lowering a carrierconcentration in the vicinity of this interface, i.e., in second p-typenitride semiconductor layer 109 which is an upper portion of the p-typeAlGaN layer and preferably in third p-type nitride semiconductor layer110 which is a lower portion of the p-type GaN layer. This vapor phasegrowth method is particularly effective in a case of interrupting thevapor phase growth between the vapor phase growth of second p-typenitride semiconductor layer 109 and the vapor phase growth of thirdp-type nitride semiconductor layer 110 by changing the pressure of thevapor phase or the like.

A nitride semiconductor light-emitting diode element according toExample 2 was prepared similarly to Example 1 except that a third p-typenitride semiconductor layer 110 was not undoped but doped with Mg andthe p-type impurity (Mg) concentration in third p-type nitridesemiconductor layer 110 was set to 4×10¹⁹ atoms/cm³.

In the nitride semiconductor light-emitting diode element according toExample 2 prepared in the aforementioned manner, the p-type impurity(Mg) concentration in a fourth p-type nitride semiconductor layer 111can be heightened to 5×10¹⁹ atoms/cm³ even at the same Cp₂Mg flow rateas in Example 1.

Thus, contact resistance between fourth p-type nitride semiconductorlayer 111 and a transparent electrode 115 was reduced in the nitridesemiconductor light-emitting diode element according to Example 2, andit was possible to reduce operating voltage for the element.

A nitride semiconductor light-emitting diode element according toExample 3 was prepared similarly to Example 1 except that TMI (trimethylindium) was further supplied as gas for vapor phase growth of a thirdp-type nitride semiconductor layer 110.

While the temperature of a template substrate during the vapor phasegrowth of third p-type nitride semiconductor layer 110 is at a highlevel of 1110° C. and hence In is not incorporated into third p-typenitride semiconductor layer 110, the rate of activation of Mg in thirdp-type nitride semiconductor layer 110 can be raised, and hence it waspossible to reduce operating voltage for the element.

As shown in FIGS. 4 and 5, an MN buffer layer 202, an undoped GaN layer203, an n-type GaN underlayer 204 (n-type impurity (Si) concentration:6×10¹⁸ atoms/cm³), an n-type GaN contact layer 205 (n-type impurity (Si)concentration: 6×10¹⁸ atoms/cm³, thickness: 1.5 μm), an n-typesuperlattice layer 206 prepared by alternately stacking. Si-doped n-typeGaN layers (n-type impurity (Si) concentration: 5×10¹⁸ atoms/cm³,thickness: 1.75 nm) and Si-doped InGaN layers (n-type impurity (Si)concentration: 5×10¹⁸ atoms/cm³, thickness: 1.75 nm) by 20 cycles, anitride semiconductor active layer 207 prepared by alternately stackingSi-doped n-type GaN barrier layers (n-type impurity (Si) concentration:4×10¹⁷ atoms/cm³, thickness: 6.5 nm) and undoped InGaN quantum welllayers (thickness: 3.5 nm) by six cycles and thereafter stacking anundoped GaN barrier layer, an undoped AlGaN layer 218 (thickness: 2 nm),a first p-type nitride semiconductor layer 208 (p-type impurity (Mg)concentration: 2×10¹⁹ atoms/cm³, thickness: 1.75 nm) made of Mg-dopedp-type AlGaN, a second p-type nitride semiconductor layer 209 (p-typeimpurity (Mg) concentration: 1×10¹⁹ atoms/cm³, thickness: 3.75 nm) madeof undoped AlGaN, a third p-type nitride semiconductor layer 210 (p-typeimpurity (Mg) concentration: 1×10¹⁹ atoms/cm³, thickness: 60 nm) made ofundoped GaN and a fourth p-type nitride semiconductor layer 211 (p-typeimpurity (Mg) concentration: 3×10¹⁹ atoms/cm³, thickness: 20 nm) made ofp⁺ GaN are stacked in this order on a sapphire substrate 201 whosesurface is irregularized, as a nitride semiconductor light-emittingdiode element 200 according to Example 4.

A transparent electrode 215 made of ITO is formed on part of the surfaceof fourth p-type nitride semiconductor layer 211, a p-side pad electrode216 consisting of a laminate of an Ni layer and an Au layer from theside close to transparent electrode 215 is formed on part of the surfaceof transparent electrode 215, and an n-side pad electrode 217 consistingof a laminate of an Ni layer and an Au layer is formed on part of thesurface of n-type GaN contact layer 205.

The nitride semiconductor light-emitting diode element according toExample 4 having the aforementioned structure was prepared as follows:

First, the elements up to nitride semiconductor active layer 207 wereprepared similarly to Example 1.

Then, the pressure of a vapor phase in an MOCVD apparatus was lowered to1×10⁴ Pa, while the temperature of a template substrate was raised to1110° C. Dispersion of Al compositions in AlGaN layers described laterbetween wafers and dispersion of Al compositions in the wafers in theMOCVD apparatus capable of growing a large number of wafers can besuppressed and the yield of elements can be improved by lowering thepressure of the vapor phase in the MOCVD apparatus to 1×10⁴ Pa.

Then, carrier gas supplied into the MOCVD apparatus was switched from N₂gas to H₂ gas for vapor-phase-growing undoped Al_(0.17)Ga_(0.83)N layer218 having the thickness of 2 nm, and first p-type nitride semiconductorlayer 208 (p-type impurity (Mg) concentration: 2×10¹⁹ atoms/cm³) made ofMg-doped p-type Al_(0.17)Ga_(0.83)N having a thickness of 11.25 nm wasthen vapor-phase-grown, while second p-type nitride semiconductor layer209 (p-type impurity (Mg) concentration: 1×10¹⁹ atoms/cm³) consisting ofan undoped Al_(0.17)Ga_(0.83)N layer having the thickness of 3.75 nm wasvapor-phase-grown.

The average Al composition in undoped Al_(0.17)Ga_(0.83)N layer 218 wasset to 17%, and no impurity source gas was fed.

The average Al composition in first p-type nitride semiconductor layer208 was set to 17%, while Cp₂Mg (biscyclopentadienyl magnesium) gas wasemployed as an Mg dopant raw material for first p-type nitridesemiconductor layer 208 and the Mg doping concentration was set to2×10¹⁹ atoms/cm³.

The average Al composition in second p-type nitride semiconductor layer209 was set to 17%, and no Cp₂Mg gas was fed during the vapor phasegrowth of second p-type nitride semiconductor layer 209.

Then, the vapor phase growth was temporarily interrupted and thepressure of the vapor phase in the MOCVD apparatus was raised to 2×10⁴Pa. Thereafter vapor phase growth was started, to vapor-phase-grow thirdp-type nitride semiconductor layer 210 (p-type impurity (Mg)concentration: 1×10¹⁹ atoms/cm³) made of undoped GaN having thethickness of 60 nm, and to vapor-phase-grow fourth p-type nitridesemiconductor layer 211 (p-type impurity (Mg) concentration: 3×10¹⁹atoms/cm³) made of p⁺ GaN having a thickness of 30 nm.

Actual impurity concentrations in the p-type layers grown in this mannerare as follows: In actually grown undoped Al_(0.17)Ga_(0.83)N layer 218,Mg is incorporated due to diffusion. While this is because Mg diffusesfrom first p-type nitride semiconductor layer 208, this Mg diffusiontakes place during the growth of first p-type nitride semiconductorlayer 208, and subsequent second p-type nitride semiconductor layer 209is so grown that the diffusion from first p-type nitride semiconductorlayer 208 into undoped Al_(0.17)Ga_(0.83)N layer 218 stops, and the Mgdiffusion is prevented by second p-type nitride semiconductor layer 209grown without feeding impurity source gas in practice during thesubsequent growth of third p-type nitride semiconductor layer 210 andfourth p-type nitride semiconductor layer 211. While the thickness ofundoped Al_(0.17)Ga_(0.83)N layer 218 is small and it is difficult todetect the Mg concentration in this layer, the Mg concentration steeplychanges in this undoped Al_(0.17)Ga_(0.83)N layer 218, the Mgconcentration in the well layer forming the uppermost layer ofnitride-semiconductor active layer 207 reaches 2×10¹⁸ atoms/cm³ due tothe presence of undoped Al_(0.17)Ga_(0.83)N layer 218, and it followsthat the Mg diffusion into the active layer has been further suppressedthan the case of Example 1.

Undoped Al_(0.17)Ga_(0.83)N layer 218 plays the role of Mg diffusionprevention, and it can be said that the same serves a function similarto that of second p-type nitride semiconductor layer 209. If onlyundoped Al_(0.17)Ga_(0.83)N layer 218 is formed and second p-typenitride semiconductor layer 209 prepared substantially in an undopedstate is eliminated, the Mg diffusion preventing function lowers. If thethickness of undoped Al_(0.17)Ga_(0.83)N layer 218 is not set to 2 nmbut further increased in order to compensate for this, the drivingvoltage for the element rises although the Mg diffusion preventingeffect rises. Therefore, it is important to keep the thickness ofundoped Al_(0.17)Ga_(0.83)N layer 218 to the minimum thickness. On thataccount, the Mg diffusion prevention and suppression of driving voltagerising can be compatibly attained by combining the same with secondp-type nitride semiconductor layer 209 prepared in the undoped state inpractice.

FIG. 6 shows atomic concentration profiles of the nitride semiconductorlight-emitting diode element (hereinafter referred to as “elementaccording to Example 4”) according to Example 4 and a nitridesemiconductor light-emitting diode element (hereinafter referred to as“element according to Example 1”) prepared by a production methodsimilar to that in Example 1 by SIMS. The axis of abscissas in FIG. 6shows depths (depth (a.u.)), and the axis of ordinates shows atomicconcentrations (concentration (a.u.)). Referring to FIG. 6, 2-Mg denotesthe Mg atomic concentration in the element according to Example 1, 3-Mgdenotes the Mg atomic concentration in the element according to Example4, Al denotes relative Al atomic concentrations (substantiallycoinciding in the element according to Example 4 and the elementaccording to Example 1), and In denotes relative In atomicconcentrations (substantially coinciding in the element according toExample 4 and the element according to Example 1).

A vertical line is drawn on a portion corresponding to the uppermostlayers of the quantum well layers of six cycles in FIG. 6, and it isassumed that Y2 and Y3 represent the Mg atomic concentration in theelement according to Example 1 and the Mg concentration in the elementaccording to Example 4 on the portion respectively. Comparing the Mgatomic concentration of Y2 and the Mg atomic concentration of Y3 witheach other, the Mg atomic concentration of Y3 reaches 3×10¹⁸ atoms/cm³while the Mg atomic concentration of Y2 is 7×10¹⁸ atoms/cm³, and it hasbeen confirmed that Mg diffusion can be further suppressed.

The driving voltage for the element according to Example 4 was 2.9 Vwith operating current of 30 mA similar to that for the elementaccording to Example 1. As to an emission output of the elementaccording to Example 4, 45 mW was obtained with operating current of 30mA.

When an electrostatic discharge threshold test of −1500 V was conductedin an HBM model as to the element according to Example 4, only 2% waselectrostatically broken in the electrostatic discharge threshold teston 30000 samples, and it has been understood that the electrostaticdischarge threshold is also excellent.

The embodiment and Examples disclosed this time must be considered asillustrative in all points and not restrictive. The range of the presentinvention is shown not by the above description but by the scope ofclaims for patent, and it is intended that all modifications within themeaning and range equivalent to the scope of claims for patent areincluded.

The present invention can be utilized for a nitride semiconductorlight-emitting element and a method for producing a nitridesemiconductor light-emitting element.

The invention claimed is:
 1. A nitride semiconductor light-emittingelement comprising: an n-type nitride semiconductor layer; a nitridesemiconductor active layer provided on said n-type nitride semiconductorlayer; and a p-type nitride semiconductor layer provided on said nitridesemiconductor active layer, wherein said p-type nitride semiconductorlayer includes a first p-type nitride semiconductor layer, a secondp-type nitride semiconductor layer and a third p-type nitridesemiconductor layer in this order from the side of said nitridesemiconductor active layer, said first p-type nitride semiconductorlayer- and said second p-type nitride semiconductor layer contain Alrespectively, an average Al composition in said first p-type nitridesemiconductor layer and an average Al composition in said second p-typenitride semiconductor layer are equivalent to each other, said thirdp-type nitride semiconductor layer has a smaller band gap than saidsecond p-type nitride semiconductor layer, and the p-type impurityconcentration in said second p-type nitride semiconductor layer and thep-type impurity concentration in said third p-type nitride semiconductorlayer are lower than the p-type impurity concentration in said firstp-type nitride semiconductor layer respectively.
 2. The nitridesemiconductor light-emitting element according to claim 1, wherein saidp-type nitride semiconductor layer further includes a fourth p-typenitride semiconductor layer on a side of said third p-type nitridesemiconductor layer opposite to the side where said nitridesemiconductor active layer is set, said fourth p-type nitridesemiconductor layer has a smaller band gap than said second p-typenitride semiconductor layer, and the p-type impurity concentration insaid fourth p-type nitride semiconductor layer is higher than the p-typeimpurity concentration in said third p-type nitride semiconductor layer.3. The nitride semiconductor light-emitting element according to claim1, wherein said nitride semiconductor active layer has a multiplequantum well structure including a plurality of nitride semiconductorquantum well layers and a plurality of nitride semiconductor barrierlayers, and a nitride semiconductor barrier layer, included in saidplurality of nitride semiconductor barrier layers, other than a nitridesemiconductor barrier layer in contact with said p-type nitridesemiconductor layer contains an n-type impurity.
 4. The nitridesemiconductor light-emitting element according to claim 1, wherein saidn-type nitride semiconductor layer includes an n-type nitridesemiconductor contact layer and an n-type nitride semiconductorsuperlattice layer, said n-type nitride semiconductor superlattice layeris positioned between said n-type nitride semiconductor contact layerand said nitride semiconductor active layer, and an average n-typeimpurity concentration in said n-type nitride semiconductor superlatticelayer is at least 1×10¹⁸ atoms/cm³.
 5. A method for producing a nitridesemiconductor light-emitting element, comprising the steps of:vapor-phase-growing a nitride semiconductor active layer on an n-typenitride semiconductor layer; vapor-phase-growing a first p-type nitridesemiconductor layer containing Al on said nitride semiconductor activelayer; vapor-phase-growing a second p-type nitride semiconductor layer,containing Al, having an equivalent average Al composition to said firstp-type nitride semiconductor layer on said first p-type nitridesemiconductor layer; and vapor-phase-growing a third p-type nitridesemiconductor layer having a smaller average Al composition than saidfirst p-type nitride semiconductor layer on said second p-type nitridesemiconductor layer, wherein said second p-type nitride semiconductorlayer and said third p-type nitride semiconductor layer are doped with ap-type impurity in lower concentrations than said first p-type nitridesemiconductor layer respectively.
 6. The method for producing a nitridesemiconductor light-emitting element according to claim 5, wherein thevapor phase growth is interrupted after the step of vapor-phase-growingsaid second p-type nitride semiconductor layer and before the step ofvapor-phase-growing said third p-type nitride semiconductor layer. 7.The method for producing a nitride semiconductor light-emitting elementaccording to claim 6, wherein the pressure of the vapor phase is changedin the interruption of said vapor phase growth.
 8. The method forproducing a nitride semiconductor light-emitting element according toclaim 5, further comprising a step of vapor-phase-growing a fourthnitride semiconductor layer doped with the p-type impurity in a higherconcentration than said third p-type nitride semiconductor layer on saidthird p-type nitride semiconductor layer after the step ofvapor-phase-growing said third p-type nitride semiconductor layer. 9.The nitride semiconductor light-emitting element according to claim 1,further comprising a nitride semiconductor layer containing Al betweensaid nitride semiconductor active layer and said first p-type nitridesemiconductor layer.